Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 87
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 87, with English translation by Maimonides (Rambam).
שלא ייזח החושן מעל האיפוד, שנאמר "ולא ייזח החושן, מעל האיפוד" (שמות כח,כח; שמות לט,כא).
For the [High Priest's] breastplate not to come loose from the ephod, as [Exodus 28:28] states: "The breastplate shall not come loose from the ephod."